The present invention relates to an electric double layer capacitor utilized as a rechargeable power supply.
A conventional electric double layer capacitor, as shown in FIG. 3, is constituted by a separator 2 for separating polarized electrodes 1a and 1b, which are formed by soaking activated carbon in a sulfuric acid solution, from each other, external electrodes 3a and 3b for connecting the polarized electrodes 1a and 1b to an external circuit, and a molding case 4 for casing them. The separator 2 consists of an insulating film such as a polyethylene film having fine pores which transmit ions but do not transmit electrons and are formed by an etching process or the like. The external electrodes 3a and 3b are acid resistant conductive electrodes consisting of carbon or the like, and the molding case 4 is an acid resistant plastic case.
In this electric double layer capacitor, when the external electrodes 3a and 3b are respectively connected to the positive and negative terminals of a power supply to charge the capacitor, the activated carbon surfaces of the polarized electrodes 1a and 1b are charged to be positive and negative, respectively. The electric double layer capacitor is a capacitor using an electric double layer formed at the interface between the activated carbon and the sulfuric acid solution serving as an electrolytic solution. Since a material such as activated carbon having a large specific surface of 1,000 m.sup.2 /g is used as an electrode, a large capacitance of 20 to 40 .mu.F/cm.sup.2 can be obtained. As a result, the capacitor which advantageously can obtain a capacitance of 200 to 400 F per gram of activated carbon. The capacitor can be extensively used in various electric circuits as a simple power supply because the capacitor can obtain a large capacitance as described above.
The charging/discharging performance of this capacitor is mainly dependent on the internal resistance thereof, i.e., a capacitor having a lower internal resistance can be quickly charged and discharged by a larger current. In the conventional electric double layer capacitor described above, the internal resistance is dependent on the number of ions which are transmitted through the fine pores of the separator 2. Therefore, although there is a state-of-the-art method of increasing the number of pores of the separator 2 or the diameter of each pore of the separator 2, the internal resistance of the separator 2 having a size of 20 cm.sup.2 is about 3 m.OMEGA., and a charging/discharging current of several amperes at most is allowed to flow in the separator 2. In addition, since the breakdown voltage of an electric double layer capacitor using a sulfuric acid solution is theoretically about 1.2 V, a large number of capacitors each shown in FIG. 3 must be connected in series with each other in accordance with a voltage to be used, and a decrease in internal resistance is a very serious problem to be solved.
In this conventional electric double layer capacitor, the magnitude of the internal resistance is dependent on the number of ions which are transmitted through the separator 2, and a required number of capacitor units for obtaining a desired breakdown voltage must be connected in series with each other to assure the breakdown voltage. For example, since the breakdown voltage of a capacitor using a sulfuric acid solution as an electrolytic solution is 1.2 V, five capacitor units must be connected in series with each other in a circuit which requires a 5-V charging/discharging circuit. In this case, since the internal resistance becomes five times that of a circuit using only one capacitor unit, the conventional capacitor unit cannot be used in a circuit having a large charging/discharging current.